TUNING IN ON "THE ANTENNAE": THE ULTRALUMINOUS STARBURST POTENTIAL

SAN DIEGO -- At a meeting of the American Astronomical Society today (June
9) radio astronomers announced that the total molecular gas content -- the
fuel for star formatin -- in a pair of colliding galaxies, known as "the
Antennae," is much greater than previously thought.

The large amount of molecular gas and the spectacular images of its very
extended and complex distribution are of special importance because they may
lead to an entirely different view of the origin and evolution of the
galactic system.

The new results were reported by Yu Gao and Robert Gruendl of the University
of Illinois, and Kwok-Yung (Fred) Lo, Chorng-Yuan Hwang, Siow-Wang Lee,
Wei-Hao Wang and Ting-Hui Lee of the Academia Sinica Institute of Astronomy
and Astrophysics in Taipei, Taiwan.

"The high value of the total molecular gas mass -- greater than 15 billion
suns -- was very unexpected because the previously accepted value was only
three billion suns," said Gao, who announced the team's findings. "The large
gas reservoir we discovered contains sufficient fuel to make the Antennae
enter an ultraluminous phase as the colliding pair continues to merge.
Eventually, the Antennae will join the ranks of the most luminous objects in
the universe."

The galaxy pair NGC 4038/4039 (also known as Arp 244), was nicknamed "the
Antennae" because of the resemblance of a pair of very long bright tails
that were formed in the collision. Although more than 65 million light-years
from Earth, the Antennae is the nearest infrared-luminous, prototypical
galaxy-galaxy merger caught in the middle of the action. Accurate
observations of the molecular gas are crucial to understanding its
star-formation history and evolution.

Detailed imaging of the distribution of molecular hydrogen -- which is
generally not visible by itself except when heated by shock waves -- was
obtained using the Berkeley-Illinois-Maryland Association (BIMA) millimeter
array in northern California. The astronomers observed the carbon monoxide
emission at a wavelength of 3 millimeters to deduce the molecular hydrogen.

The discovery of the large molecular gas mass was based on a careful mapping
of the carbon monoxide emission using the 12-meter radio telescope at the
National Science Foundation's National Radio Astronomy Observatory in
Arizona. The previously accepted value for the total molecular mass had been
obtained from a single pointing observation made with the same telescope
more than a decade ago. Because this galaxy system is much larger than the
telescope's beam, the original observation detected only a small part of the
total emission. A detailed mapping of the system's extended carbon monoxide
emission was essential for more accurately estimating the total molecular
gas.

In addition, the interferometric data from the 10-element BIMA array were
combined with the 12-meter, single-dish data to produce a fully sampled
synthesis image. "This not only recovered all the extended emission in the
system but also gave the best linear resolution scale of about 1,000
light-years, comparable to the size of large associations of giant molecular
clouds found in our own galaxy," Gruendl said. "These observations provide
clues to the origin and evolution of the Antennae."

The Antennae has a total infrared luminosity of nearly 100 billion suns,
barely enough to be classified as a luminous infrared galaxy. Such galaxies
are believed to be primarily the result of collisions and mergers of
gas-rich spiral galaxies, which lead to a strong enhancement of new star
formation.

"Events like these may have been much more numerous in the early universe
when galaxy collisions were more frequent," Gao said. "Normal gas-rich
spiral galaxies are the building blocks of the luminous infrared galaxies.
Late-stage mergers tend to be ultraluminous with an infrared power of more
than 1,000 billion suns, comparable to the bolometric luminosity of quasars,
the most luminous objects in the universe."

With such a large quantity of raw material available for star formation, the
Antennae has the potential to enter an ultraluminous starburst phase in the
future, Gao said.

There is a wealth of recent observations of the Antennae, including the
Hubble Space Telescope optical images, the Infrared Space Observatory
mid-infrared images and numerous observations made in essentially all
astronomical windows from radio to X-ray. "But much of our understanding,
interpretation and modeling have been derived from the much lower molecular
gas mass determined more than a decade ago," Gao said. "It is important to
reconsider all of these observations accordingly."

Currently, there is no existing facility capable of mapping the far-infrared
emission of the Antennae at a high-enough resolution, Gao said. "However,
radio continuum emission at centimeter wavelengths mapped by the Very Large
Array in New Mexico can show roughly the same structure because the two are
tightly correlated. We have obtained high-resolution Very Large Array data
and it shows very good correspondence to our BIMA carbon monoxide map."

The carbon monoxide images of the Antennae, as well as images of another
dozen more distant colliding/merging luminous infrared galaxies, were
obtained by Gao and Gruendl and their team over the last few years as one of
BIMA's key projects. The goal is to statistically study an entire merger
sequence to trace the merging progress and the starburst development by
mapping the detailed distribution and kinematics of the star-forming
molecular gas.

COLOR PICTURES

The BIMA carbon monoxide images and the Very Large Array radio continuum
images are available on the World Wide Web.
The first picture is a full synthesis carbon monoxide image showing the
distribution of molecular gas -- the fuel for star formation -- in the
Antennae. The image was made by combining interferometric data from the
10-element BIMA array in northern California with single-dish data from the
12-meter radio telescope at the National Radio Astronomy Observatory in
Arizona. The background false-color image is the Hubble Space Telescope's
optical image for comparison.

Photo Credit: University of Illinois and Berkeley-Illinois-Maryland
Association

The second picture shows a fully sampled carbon monoxide observation made at
a half-telescope-beam spacing with the 12-meter radio telescope at the
National Radio Astronomy Observatory in Arizona. More than 70 positions have
been observed (as indicated by red dots). Because the Antennae is much
larger than the telescope's beam, the original single-pointing observation --
made over a decade ago -- detected only a small part of the total emission. A
detailed mapping of the extended carbon monoxide emission was essential for
more accurately estimating the total molecular gas content.

Photo Credit: Yu Gao (University of Illinois)

The third picture shows the radio continuum emission at a wavelength of 20
centimeters obtained with the Very Large Array in New Mexico. The emission
contours have been overlaid on a false-color, ground-based optical image of
the Antennae.